The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In most instances, the steam generators that are required for a piston steam engine consist of a heat transfer device within which the working medium, for example water, is vaporized at the desired operating pressure. The heat that is required for the vaporization process is generated by a thermal-transfer medium, for example smoke gases. Within the steam generator, the thermal transfer medium is cooled to a temperature within the range of the vaporization temperature of the working medium.
In another approach, the attempt is made to bring about so-called flash vaporization in a screw-type machine. Here, reference is made to the work of Professor Kauder of the University of Dortmund. The principle disadvantages of a screw-type machine are numerous.
In a screw-type machine, the compression or expansion ratios (subsequently referred to as the volume ratio) are between approximately 4 and a maximum of 8. In a piston steam engine, volume ratios of greater than 100 can be achieved.
The convective heat exchange that takes place between the working medium and the walls of the screw-type machine is extremely large because there exists a fully formed two-phase flow, besides which the heat-transferring surface is very large.
By virtue of its construction, the degree of efficiency of a screw-type machine is relatively low, and the leakage losses cannot be reduced by seals or piston rings, as is the case with piston steam engines.
In the case of other known combustion engines that are on the market, for example conventional piston steam engines, ORC engines that operate according to the organic Rankine cycle, Rankine engines, or steam turbines, only a relatively low mechanical performance can be achieved from them, particularly if the heat source is at a relatively low temperature, for example 200° C.
In order to make the best possible use of the energy contained in the heat of the working medium, the heat-transfer medium of the heat source should be cooled to ambient temperature in a process that is as reversible as possible.
In general, however, in the steam generators of known combustion engines, the thermal transfer of the heat source is cooled only to a temperature that is close to the vaporization or condensation temperature. For example, the thermal transfer medium is cooled only from 200° C. to 140° C. and not to ambient temperature. In particular, if only heat at a relatively low temperature level is available, and only a small amount of it is convertible into mechanical energy, then this relatively high end temperature of the thermal transfer medium of the heat source and the associated low exergonic efficiency has a particularly deleterious effect on the performance and the economics of the combustion engine.
In addition, partially toxic or injurious working media are used in many of the combustion engines referred to above.